Microencapsulation is utilized to change the apparent state and properties of encapsulated core material, protect the material in a finely-divided form, control its release, and release the contents at the time desired.
In recent years, microcapsules have been used for image recording materials, medicines, perfumes, agricultural chemicals, adhesives, foods, detergents, dyes, solvents, catalysts, enzymes and rust inhibitors, specific examples being pressure-sensitive copying paper, aspirin capsules, perfume capsules, pressure-sensitive capsule adhesives, active charcoal capsules, enzyme capsules, liquid crystal capsules, and methylparathion capsules.
A wide variety of processes exist by which microcapsules can be manufactured. These varied processes provide different techniques for producing capsules of varying sizes, alternative materials for the composition of the capsule shell and various different functional materials within the shell. Some of these various processes are shown in U.S. Pat. Nos. 3,516,846; 3,516,941; 3,778,383; 4,087,376; 4,089,802; 4,100,103 and 4,251,386 and British Patent Specification Nos. 1,156,725; 2,041,319 and 2,048,206. A popular material for shell formation is the polymerization reaction product between urea and formaldehyde or melamine and formaldehyde, or the polycondensation products of monomeric or low molecular weight polymers of dimethylurea or methylated dimethylurea with aldehydes. A variety of capsule forming materials are disclosed, for example, in U.S. Pat. Nos. 3,516,846 and 4,087,376 and U.K. Patent Specification Nos. 2,006,709 and 2,062,570.
As shown in these references, the principal utility of microencapsulated materials is in the formation of a surface coated with the microcapsules in a binder. The microcapsules are ruptured by various means to release the material contained therein. In addition to release of physically observable materials such as ink in order to form a visible image, other types of active ingredients such as odor releasing materials, bacteriostatic materials, chemically active materials and the like have been provided in this manner.
Printed articles capable of undergoing either plastic or inelastic deformation, but without an applied layer of microscopic rupturable capsules, are known in the art, such as printed "silly putty".
Web, film and sheet materials coated with a microcapsule containing layer, where the release of the liquid phase, vaporizable solid, or solid dispersed in a liquid is stimulated by pressure or by shear applied to the microcapsule layer, e.g., by scratching or scraping with a fingernail, are also known in the art. These are available, for example, as scratch `n` sniff stickers.